(1) Field of the Invention
The present invention relates generally to telemetry, and more particularly to telemetry of multiple data channels.
(2) Description of the Prior Art
In many types of tests it is necessary to collect data in one location and transmit that data to another location for processing. For example, a sensor may be placed in a hostile environment and monitored by personnel at a distant, safe location. A channel is established between the sensor and the monitor over which the data is carried. Typically, however, data collection requires the use of multiple sensors and multiple channels. Providing separate carriers for each channel quickly becomes too costly as the number of sensors increases. Various means have been developed to provide separate data channels within a single carrier. Two of the more common telemetry schemes include frequency-division multiplexing (FDM) and time-division multiplexing (TDM). In FDM, each data channel signal is used to modulate a carrier signal having a unique frequency and the carrier signals are multiplexed by summing them together. At the signal monitoring end, receivers responsive to the separate frequencies can retrieve the individual data channel signals from the multiplexed carrier signal. In TDM, each data channel signal is allotted a time slot. To obtain a specific data signal, the receiver selects the time slot for that data signal. Various means can be used in FDM and TDM. U.S. Pat. No. 5,406,553 to Smith et al., recites an apparatus for converting a FDM signal to a TDM signal in which a voltage control oscillator whose output is a staircase of reference frequencies is mixed by a mixer with a received FDM signal. Each received frequency is output in turn at a common intermediate frequency from a bandpass filter, thereby forming a TDM signal after passing through a demodulator and low pass filter. U. S. Pat. No. 5,608,724 to Green, Jr., recites a dynamically reconfigurable FDM and modulator which accepts multiple TDM input signals. Each input signal is used to modulate a carrier signal having a unique frequency. Switches are used to selectively form one or more groups of the modulated signals and the signals in each group are FDM'd. The output for each group is fed to a respective antenna and the switches and carrier signals are controlled to selectively associate input signals, carrier signals and antennas. However, such multiplexing systems become costly and tend to be complex and bulky. In some testing environments, there is limited space for the sensors and receivers and the use of the described prior art systems may not be practical. Also, these prior art systems typically have limited dynamic range, bandwidth channel capacity and sensitivity.